This invention relates to novel radiation-activatable photocrosslinking agents. This invention also relates to radiation-crosslinkable elastomers. This invention further relates to radiation-crosslinked elastomners.
It is known that crosslinking of polymers produces polymer networks which have quite different mechanical and physical properties compared to their uncrosslinked linear or branched counterparts. For example, polymer networks can show such unique and highly desirable properties as solvent resistance, high cohesive strength, and elastomeric character.
Crosslinked polymers can be made in situ during formation of the desired polymer product, however, since further processing of the polymer product is often necessary, it is more typical to start from the linear or branched polymer which in the final processing step is cured to a crosslinked material. The curing or crosslinking step is typically activated by moisture, thermal energy, or radiation. The latter has found widespread applications, particularly in the use of ultraviolet light as the radiation source.
In the past, a variety of different materials have been used as crosslinking agents, e.g., polyfunctional acrylates, acetophenones, benzophenones, and triazines. The foregoing crosslinking agents, however, possess certain drawbacks which include one or more of the following: high volatility; incompatibility with certain polymer systems; generation of corrosive or toxic by-products; generation of undesirable color; requirement of a separate photoactive compound to initiate the crosslinking reaction; and high sensitivity to oxygen.
Certain polyfunctional benzophenones have been investigated as photocrosslinking agents and/or photosensitizers in various photopolymerizable systems.
JP 54/057560 discloses the use of (bis)benzophenone compounds to photocrosslink non-elastomeric materials in particular, polyester compositions. When incorporated into polyesters, they impart improved tensile strength and elongation to biaxially stretched films of crosslinked poly(ethylene terephthalate). These films also exhibit enhanced weather, heat, and chemical resistance and improved dimensional stability.
U.S. Pat. No. 4,602,097 (Curtis) discloses the use of (bis)benzophenones as photoinitiators and/or photosensitizers in radiation-cured coatings. The poly(ethylene oxide) moiety which separates the terminal benzophenone groups allows the claimed compositions to be more soluble than unsubstituted-benzophenones in waterborne coating compositions. The (bis)benzophenone compounds, however, contain hydrogen donating groups, such as the methylenes adjacent to the oxygen atoms of the ether functionalities. These hydrogen donating groups undergo an intramolecular hydrogen abstraction by the photochemically excited (bis)benzophenone structure to provide a lower energy radical which is effective as an initiator, but unsuitable as a photocrosslinker.
PCT Patent Appln. WO 93/16131 and U.S. Pat. No. 5,407,971 (Everaerts et al.) describes a radiation-crosslinkable elastomeric composition containing: (a) an elastomeric polymer containing abstractable hydrogen atoms in an amount sufficient to enable the elastomeric polymer to undergo crosslinking in the presence of a suitable radiation-activatable crosslinking agent; and (b) a radiation-activatable polyfunctional acetophenone or benzophenone crosslinking agent. According to Formula (1) of this application, if substituent xe2x80x9cWxe2x80x9d is present (i.e., the aceto- or benzophenone moieties of these crosslinkers have an ether, thioether or amino linkage), then an internal ketone, ester or amide functionality (i.e., substituent xe2x80x9cYxe2x80x9d) must also be present. From a synthetic standpoint, such crosslinkers are prepared in a reaction sequence involving at least two steps. The first step involves preparation of an acetophenone- or benzophenone-functional alkyl ester derivative. The second step involves the reaction of this of this alkyl ester with either short chain or higher molecular weight nucleophiles. Additional reaction steps may also be required if other functionalities, such as urethane groups, are desired in spacer xe2x80x9cZxe2x80x9d.
U.S. Pat. No. 4,379,201 (Heilmann et al.) is an example of a class of polyacrylic-functional crosslinkers used in the photocuring of (meth)acrylate copolymers. U.S. Pat. No. 4,391,678 (Vesley) and U.S. Pat. No. 4,330,590 (Vesley) describe a class of fast curing triazine photocrosslinkers which, when mixed with an acrylic monomer and, optionally, a monoethylenically unsaturated monomer, and exposed to UV radiation, forms a crosslinked polyacrylate. The crosslinks formed by both the (meth)acrylates and the triazines in these copolymerizations prevent any further processing, such as hot melt coating, reactive extrusion, or solution coating processes, following the initial photopolymerization.
U.S. Pat. No. 4,737,559 (Kellen et al.) discloses acrylate-functional aromatic ketones (in particular, 4-acryloxybenzophenone xe2x80x9cABPxe2x80x9d) which are incorporated with other (meth)acrylate monomers to form pressure-sensitive adhesive copolymers containing pendant benzophenone groups. These benzophenone functional pressure-sensitive adhesive copolymers undergo efficient crosslinking upon exposure to UV light, especially when compared to the use of conventional benzophenones as a photocrosslinker. This patent also specifically states that the disclosed compounds must be free of hydroxy groups in a position ortho to the carbonyl functionality. These hydroxy substituents inhibit free-radical formation and hydrogen abstraction from the acrylate copolymer backbone. However, since these acrylate-functional aromatic ketones are monomers to be copolymerized primarily with other acrylic monomers, they are not usefull as a post-polymerization photocrosslinker which may be compounded with previously prepared elastomeric polymers of varying chemical character.
There is a strong desire to be able to crosslink adhesive systems after all processing requirements have been accomplished. As the industry moves towards the use of hot-melt adhesives and away from solvent-based coatings, this requirement becomes even more important. Many approaches and polymer types have been studied to obtain the desired properties. E-beam and UV radiation curing have been leading the way with respect to post-radiation curing. There are problems associated with both routes and no universal solution is currently apparent.
It was against the foregoing background that a search for improved radiation-crosslinkable materials and radiation-activatable crosslinking agents was conducted.
In accordance with one embodiment of the present invention, there is provided a radiation-crosslinkable composition comprising: (a) an elastomeric polymer containing abstractable hydrogen atoms in an amount sufficient to enable the elastomeric polymer to undergo crosslinking in the presence of a suitable radiation-activatable crosslinking agent; and (b) a radiation-activatable crosslinking agent of the formula: 
wherein:
X represents CH3xe2x80x94; phenyl; or substituted-phenyl with the proviso that any substituents on the substituted-phenyl do not interfere with the light-absorbing capacity of the radiation-activatable crosslinking agent and do not promote intramolecular hydrogen abstraction of the radiation activatable crosslinking agent;
W represents xe2x80x94Oxe2x80x94,xe2x80x94NHxe2x80x94, or xe2x80x94Sxe2x80x94;
Z represents an organic spacer selected from the group consisting of aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic groups free of esters, amides, ketones, and urethanes, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms intramolecularly accessible to the carbonyl group in formula (I); and
n represents an integer of 2 or greater; preferably 2-6.
It is also within the spirit and scope of the present invention that the phenylene ring of formula (I) linking a carbonyl group and xe2x80x9cWxe2x80x9d can also contain one or more substituents which do not interfere with the light-absorbing capacity of the crosslinking agent and which do not promote intramolecular hydrogen abstraction of the elastomer.
It is also within the spirit and scope of the present invention that the organic spacer Z of formula (I) may contain a minimal number of esters, amides, ketones, and urethanes within its internal structure, and not as terminal groups, which contain some abstractable hydrogen atoms, yet do not lead to xe2x80x9cintramolecular backbitingxe2x80x9d of the radiation-activatable crosslinking agent formula (I).
In one preferred embodiment of the present invention, the radiation-activatable crosslinking agent used in the radiation-crosslinkable elastomer is of the formula (I) above wherein X is phenyl; W is oxygen; Z is "Parenopenst"CH2"Parenclosest"2-12; and n is 2.
In another preferred embodiment of the present invention, the radiation-activatable crosslinking agent used in the radiation-crosslinkable elastomer has the formula (II) shown below: 
wherein: Y represents carbon or phosphorus, each R substituent independently represents hydrogen; C1 to C6 alkyl; C1 to C6 alkoxy; or halogen; and x is 1 or 2, with the proviso that when Y is carbon, x must equal 1 and when Y is phosphorus, x must equal 2.
So far as is known, no one has previously utilized any of the above-disclosed radiation-activatable polyfunctional acetophenones and benzophenones as crosslinking agents for elastomeric polymers. Additionally, the use of the above-disclosed polyfunctional acetophenones and benzophenones affords a number of advantages as compared to the use of conventional crosslinking agents for elastomers. These advantages include, but are not limited to, lowered volatility of the crosslinking agent due to its higher molecular weight; increased compatibility of the crosslinker through the selection of the organic spacer; decreased sensitivity of the crosslinkable composition to oxygen; the avoidance of evolution of any toxic or corrosive by-products or discoloration of the final product; and the capability to be used as a post-curing crosslinking additive. Furthermore, the crosslinking agents for elastomeric polymers of the present invention have the following advantages over previously described polyfunctional acetophenones and benzophenones; ease of synthesis; improved crosslinking efficiency; lower cost starting materials; and optional inclusion of substitution on the benzophenone group.
The classes of radiation-activatable crosslinkers represented by formula (II) are 2,4,6-tri(4-benzoylphenoxy)-1,3,5-triazines and hexakis(4-benzoylphenoxy)-1,3,5-phosphazenes which can be synthesized in one step from commercial starting materials. The UV-visible spectra of these multifunctional benzophenone photocrosslinkers generally have greater range and extinction coefficients greater than conventional benzophenones. They are non-volatile, non-HCl producing, non-photoyellowing, and photocrosslink under both high and low intensity UV light.
In another embodiment of the present invention, novel photoactivatable crosslinkers are provided. They are 2,4,6-tri(4-benzoylphenoxy)-1,3,5-triazines based upon formula (II) wherein Y represents carbon, x is 1, and R is as defined previously.
Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, the examples, and the claims.
The radiation-crosslinkable compositions used in the present invention are elastomeric polymers (xe2x80x9celastomersxe2x80x9d) which contain abstractable hydrogen atoms. The abstractable hydrogen atoms will be present in the backbone and/or side chains of the elastomer in an amount sufficient to allow crosslinking of the elastomer upon exposure of the photocrosslinkinig agent/elastomer mixture to radiation, e.g., electromagnetic radiation, such as ultraviolet (xe2x80x9cUVxe2x80x9d) light. As a general rule, hydrogen atoms are most easily abstracted from tertiary carbon atoms, allylic and benzylic groups, those hydrogens on carbon atoms in a position alpha to an oxygen or nitrogen atom (e.g., organic ethers and tertiary amines), and those elastomners with terminal or pendant mercapto groups.
In the present invention, an elastomeric polymer or elastomer is defined as being a macromolecular material that returns rapidly to its approximate initial dimensions and shape after substantial deformation by a weak stress and subsequent release of that stress as measured according to ASTM D 1456-86 (xe2x80x9cStandard Test Method For Rubber Property-Elongation At Specific Stressxe2x80x9d). Examples of elastomers which can be used in the present invention include, but are not limited to, styrene-butadiene rubber (SBR), styrene-isoprene-styrene block copolymers (SIS), styrene-butadiene-styrene block copolymers (SBS), ethylene-propylene-diene monomer rubbers (EPDM), polyisobutylene, natural rubber, synthetic polyisoprene, polybutadiene, acrylonitrile-butadiene copolymers, polychloroprene, ethylene-vinylacetate, poly(xcex1-olefins), poly(vinyl ethers), poly(vinyl esters), polymethacrylates, and polyacrylates. The preferred elastomers for use in the present invention are polyacrylates, natural rubber, polybutadiene, polyisoprene, SBS block copolymers, and SIS block copolymers.
The radiation-activatable crosslinking agents utilized in radiation-crosslinkable elastomer of the present invention have the following formula: 
wherein:
X represents CH3xe2x80x94; phenyl; or substituted-phenyl with the proviso that any substituents on the substituted-phenyl do not interfere with the light-absorbing capacity of the radiation-activatable crosslinking agent and do not promote intramolecular hydrogen abstraction of the radiation-activatable crosslinking agent;
W represents xe2x80x94Oxe2x80x94,xe2x80x94NHxe2x80x94, or xe2x80x94Sxe2x80x94;
Z represents an organic spacer selected from the group consisting of aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic groups free of esters, amides, ketones, urethanes, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms intramolecularly accessible to the carbonyl group in formula (I); and
n represents an integer of 2 or greater; preferably 2-6.
Substituents on any phenyl or phenylene rings of formula (I) which would interfere with the light-absorbing capacity of the radiation-activatable crosslinking agent are those which are chromophoric in nature and absorb light in the range of about 240 to 400 nm and preferably, about 290-350 nm, with extinction coefficients larger than the corresponding absorptions in unsubstituted Formula (I). Examples of non-light absorbing substituents include halogen, alkoxy, and alkyl substituents.
Phenyl or phenylene substituents in formula (I) should also be free of intramolecularly accessible, readily abstractable hydrogens which are present in such functionalities as ethers, thiols, allylic groups, benzylic groups, tertiary amines, and the like to prevent or limit the incidence of deleterious intramolecular reactions.
The foregoing crosslinking agents of formula (I) can be synthesized according to reactions well known to those skilled in the art of synthetic organic chemistry, e.g., an SN2 nucleophilic aliphatic substitution reaction between 4-substituted-benzophenone, 4-substituted-acetophenone, or derivatives thereof with halofunctional aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic compounds free of urethanes, esters, amides, ketones, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms intramolecularly accessible (defined herein later) to the carbonyl group in formula (I).
Organic spacer segments Z and phenyl or phenylene substituents in formula (I) may be prepared to enhance the compatibility and decrease the volatility of the polyfunctional photocrosslinking agents in varying polymeric systems. For example, organic spacer segment Z and phenyl or phenylene substituents in formula (I) can be selected to enhance the aliphatic character of the typically aromatic benzophenone or acetophenone moieties. Such modification can result in photocrosslinking agents which are more compatible and efficient in elastomeric materials such as natural rubber, polybutadiene, poly(xcex1-olefins), and the like.
The organic spacer segment Z may also be selected to modify the rheological and mechanical properties of the radiation-crosslinked materials. A rigid spacer group will result in a different rheology than a flexible spacer group. Also, the length of the spacer group may be used to control the crosslink density of the network. Although the spacing of the crosslinking points along the backbone of the elastomer may not be precisely controlled, the size and chemical nature of the linkage may be determined using the crosslinking agents disclosed herein. As the concentration of crosslinking agent decreases in the photocurable mixture, the properties of the crosslinked elastomeric network become increasingly dominated by the mechanical and rheological properties of the elastomer.
Organic spacer Z should be free of such functionalities as ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms which are intramolecularly accessible to the carbonyl group in formula (I). xe2x80x9cIntramolecular accessibilityxe2x80x9d relates to the steric, orientational and/or conformational ability of the excited carbonyl group in formula (I) to approach closely enough to the hydrogen atoms to effect the abstraction process. When such functionalities are present, irradiation will cause hydrogen abstraction at sites along the spacer segment instead of abstracting hydrogens from the elastomeric polymer backbone. This leads to an undesired intramolecular xe2x80x9cbackbitingxe2x80x9d reaction which reduces the photocrosslinking efficiency of multifunctional crosslinkers which contain spacer segments with readily abstractable hydrogens.
In one preferred embodiment of the present invention, the radiation-activatable crosslinker used in the radiation-crosslinkable composition is of the formula (I) wherein: X is phenyl; W is oxygen; Z is "Parenopenst"CH2"Parenclosest"2-12; and n is 2.
In another preferred embodiment of the present invention, the radiation-activatable crosslinkers used in the radiation-crosslinkable elastomeric composition are of the formula (II) shown below: 
wherein: Y represents carbon or phosphorus, each R substituent independently represents hydrogen; C1 to C6 alkyl; C1 to C6 alkoxy; or halogen; and x is 1 or 2, with the proviso that when Y is carbon, x must equal 1 and when Y is phosphorus, x must equal 2.
Novel compounds according to formula (II) wherein Y represents carbon; x is 1; and R is defined as previously, are also provided by the present invention.
The compounds of formula (II) can be synthesized in at least three ways, although the routes vary in convenience and yield. 4-hydroxybenzophenone can be treated with 2,4,6-trichlorotriazine in the presence of potassium carbonate in refluxing xylenes to give moderate yields of the parent compound, which is readily recrystallized from toluene/ethyl acetate. A similar pathway that uses pyridine as both base and solvent gives fair yields of the fluoro-substituted compound. The simplest route is in situ formation of cyanogen bromide from bromine and sodium cyanide, addition of the 4-hydroxybenzophenone and triethylamine to give the aryl cyanate, and subsequent heating to yield the triazine. Furthermore, the addition of Lewis acids (e.g., TiCl4) can be employed to accelerate the trimerization of the cyanate. This one-pot synthesis is tolerant of a variety of functionalities and provides good yields.
4-hydroxybenzophenones are available commercially (e.g,. Aldrich) and/or can be prepared by literature methods.
Preferably, about 0.01-25 weight % photocrosslinking agent, more preferably, about 0.1-10 weight %, and most preferably about 0.1-1.0 weight %, is employed based upon the total weight of the elastomer. In general, the amount of photocrosslinking agent employed is based upon the ease of hydrogen abstraction from the elastomeric polymer backbone, the reactivity of the radicals formed, the intensity and length of exposure of the composition to irradiation, and the elastomer""s molecular weight and the desired final properties of the material.
Other useful materials which can be optionally utilized in the present invention include, but are not limited to, thermally expandable polymeric microspheres, glass microspheres, fillers, pigments, foaming agents, stabilizers, fire retardants, and viscosity adjusting agents which do not interfere with crosslinking.
In practice, the photocrosslinking agent and other ingredients are added to the elastomer, whereupon the material can be coated by methods well-known in the art, such as solvent coating, hot-melt coating, solventless or waterborne coating, and extrusion. The coating is then exposed to radiation, preferably electromagnetic radiation such as UV light, under conditions sufficient to effect crosslinking of the elastomer.
The photocrosslinkers of Formula (I) are preferably activated with long wavelength ultraviolet radiation (240-400 nm). The absorption maximum will depend on the molecular structure of the photocrosslinking agent. High intensity UV lights are preferably used for curing. Such UV lights, including the PPG UV processor and Fusion Systems curing unit, are commercially available. The PPG UV processor is equipped with two medium pressure mercury lamps which have a spectral output between 240 and 740 nm with emissions primarily in the 270 to 450 nm output range. The lamps can be set at full power (300 Watts/inch) or half power (150 watts/inch). The Fusion Systems Curing Unit uses UV lamps having a power supply of 300 watts/inch. A variety of bulbs are available with differing spectral outputs. The preferred bulbs for the photocrosslinking agents of the invention are the xe2x80x9cDxe2x80x9d or xe2x80x9cHxe2x80x9d bulbs, both commercially available from Fusion Systems Corp., Rockville, Md.
The radiation-crosslinked materials of the present invention are useful as sealants and coating materials, such as inks, adhesives, printing and photographic coatings, paints, semiconductor masks, release coatings, photoresists, and photodetackifiable adhesives.
The following test procedures were used to evaluate the pressure-sensitive materials used in the examples.
Peel Adhesion
Peel adhesion is the force required to remove a coated flexible sheet material from a test panel measured at a specific angle and rate of removal. In the examples, this force is expressed in Newtons per decimeter (N/dm) width of coated sheet. The test follows the procedures found in ASTM D 3330-87 (xe2x80x9cPeel Adhesion of Pressure Sensitive Tape at 180xc2x0 Anglexe2x80x9d). The only deviations from the ASTM test are the substitution of a glass plate for the steel plate called for in the test and a change in the peel rate. A glass test plate is washed with diacetone alcohol and cleaned with an absorbing material, such as a paper towel. The plate is then dried and washed three more times with heptane. A strip 0.127 dm in width of the sheet coated with the adhesive to be tested is applied to the horizontal surface of the cleaned glass test plate with at least 1.27 lineal dm in firm contact. Three passes in each direction with a 2 kg hard rubber roller is used to apply the strip. If air bubbles are entrapped between the test plate and the test strip, then the sample is discarded. The free end of the coated strip is doubled backed nearly touching itself so the angle of removal will be 180xc2x0. The free end is attached to the adhesion tester scale. The glass test plate is clamped in the jaws of a tensile testing machine which is capable of moving the plate away from the scale at a constant rate of 2.3 meters per minute. The dwell time after roll down is 30 seconds. The scale reading in Newtons is recorded as the tape is peeled from the glass surface. The data for the first 0.5 dm of the strip is disregarded and the peak, valley, and average peel is recorded for the remainder of the test strip.
Shear Strength
The shear strength is a measure of the cohesiveness or internal strength of an adhesive. It is based upon the amount of force required to pull an adhesive strip from a standard flat surface in a direction parallel to the surface to which it has been affixed with a definite pressure. It is measured in minutes required to pull a standard area of adhesive coated sheet material from a stainless steel test panel under stress of a constant, standard load. This test follows the procedure described in ASTM D 3645M-88: xe2x80x9cHolding Power of Pressure Sensitive Adhesive Tapes.xe2x80x9d
The tests were conducted on strips of coated sheet material applied to a stainless steel panel which was cleaned and prepared as described above. A 0.127 dm by 0.127 dm portion of each strip was in firm contact with the panel with one end portion of the tape being free. The panel with the coated strip attached was held in a rack such that the panel formed an angle of 178xc2x0 with the extended tape free end which was tensioned by application of a force of 1000 grams applied as a hanging weight from the free end of the coated strip. The 2xc2x0 less than 180xc2x0 is used to negate any peel forces, thus insuring that only the shear forces are measured, in an attempt to more accurately determine the holding power of the tape being tested. The time elapsed for each coated film to separate from the test panel was recorded as the shear strength. The type of failure, either xe2x80x9cadhesivexe2x80x9d failures when the adhesive separates cleanly from the panel or backing, or xe2x80x9ccohesivexe2x80x9d failures in which the sample adhesive leaves residue on both the test panel and backing, is recorded.
Gel Fraction
A known amount of polymer was put in an excess of a solvent capable of dissolving the polymer and allowed to dissolve over a 24 hr period. The sample was filtered and the recovered solid was washed a couple times with fresh solvent. The solid was dried and the amount recorded. The gel content was determined as follows:             solid      ⁢              xe2x80x83            ⁢      weight              initial      ⁢              xe2x80x83            ⁢      weight      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      sample        xc3x97  100  ⁢  %